1. Field of the Invention
The present invention relates to microwave field modifiers, and more particularly, to such modifiers which generate a significant amount of heat, i.e., susceptors. Specifically, the present invention relates to susceptors consisting of an electrically continuous coating material coated on a dielectric substrate.
2. Description of the Prior Art
Microwave ovens possess the ability to heat, cook or bake items, particularly foodstuffs, extremely rapidly. Unfortunately, microwave heating also has its disadvantages. For example, microwave heating alone often fails to achieve such desirable results as evenness, uniformity, browning, crispening, and reproducibility. Contemporary approaches to achieving these and other desirable results with microwave ovens include the use of microwave field modifying devices such as microwave susceptors.
Generically, microwave susceptors are devices which, when disposed in a microwave energy field such as exists in a microwave oven, respond by generating a significant amount of heat. The susceptor absorbs a portion of the microwave energy and converts it directly to thermal energy which is useful for example to crispen or brown foodstuffs. This heat may result from microwave induced intramolecular or intermolecular action; from induced electrical currents which result in so called I.sup.2 R losses in electrically conductive devices (hereinafter referred to as ohmic heating); or from dielectric heating of dielectric material disposed between electrically conductive particles, elements or areas (hereinafter alternatively referred to as fringe field heating or capacitive heating).
In any event the microwave susceptor absorbs a portion of the microwave energy within the oven cavity. This absorption reduces the amount of microwave energy available to cook the food. Simultaneously, the susceptor makes thermal energy available for surface cooking of the food by conductive or radiant heat transfer. Thus, susceptors tend to slow down direct microwave induction heating to provide some thermal heating which tends to be more uniform and provide such desirable results as browning or crispening.
Currently, the most commercially successful microwave susceptor is a thin film susceptor which heats through the I.sup.2 R mechanism resulting in ohmic heating. Typically, thin film susceptors are formed of a thin film of metalized aluminum vacuum deposited on a polyester layer which is adhered to paper or cardboard. This type of susceptor has its limitations. For example, these thin film susceptors provide only moderate heating performance. They do not generate the high heating performance necessary to brown or crispen high moisture content foods. More importantly, thin film susceptors are expensive to manufacture and lack the versatility and manufacturing cost advantages that coating materials offer.
Various other microwave susceptors have been proposed but have not been as commercially successful. A large number of these susceptors employ graphite or carbon as the microwave active particle. Although some of these susceptors can reach high temperatures, they tend to suffer from either runaway heating or significant degradation. Runaway heating occurs when such high power is generated over the heating cycle that the temperature rises above desirable limits. Significant degradation occurs when the susceptor degrades during the cooking cycle reducing heat output such that all conduction cooking virtually ceases. Examples of such suceptors are disclosed in U.S. Pat. No. 4,640,838 issued to Isakson et al., on Feb. 3, 1987, U.S. Pat. No. 4,518,651 issued May 21, 1985 to Wolfe, Jr., and U.S. Pat. No. 4,959,516 issued to Tighe et al., on Sep. 25, 1990.
As another example, U.S. Pat. No. 4,190,757 issued to Turpin et al. on Feb. 26, 1980 discloses a microwave package. The package includes a susceptor made up of a preferably metal substrate and a relatively thick dry layer. The dry layer is made up of a binder containing a lossy material. Sodium silicate is mentioned as a binder and such things as semiconductors, ferromagnetic materials, carbon or graphite are suggested as the lossy material.
It is believed that the present invention offers a unique combination of benefits. The susceptor of the present invention is capable of reaching extremely high temperatures. This enables it to cook foods which heretofore did not favorably brown and crispen in the microwave oven. Moreover, the susceptor can be formulated such that when a maximum temperature is reached the susceptor shuts down which avoids runaway heating. This can be important for example, if inexpensive but ignitable substrates such as paper are desired; particularly if a temperature near the ignition point is desired for effective cooking. Furthermore, although these high temperatures can be reached, the mass of the susceptor can be small to allow quick cooling avoiding possible injury.